Nukes are not cheap. It is cheaper to firebomb. I would love if the reason nukes were not used was that of empathy or humanitarian. It is strictly money, optics, psychological and practicality.

You don't want your troops to have to deal with the results of a nuked area. You want to use the psychological terror to dissuade someone to invade you, while you are invading them or others. See Russia's take.

Or you are a regime and want to stay in power. Having them keeps you in power; using them or crossing the suggestion to use them line will cause international retaliation and your removal. (See Iraq.)